There are several criteria which a suitable high temperature conductivity electrode should meet. First, the cell constant should vary only slightly with temperature. Furthermore, the temperature dependence of the cell constant should be a well-defined and smoothly varying function. For these criteria to be met, there are certain properties which the electrode should have. For example, electrode geometry should change very slightly and predictably over a large temperature range (100-350.degree.C.). In addition, electrode response should exhibit no thermal hysteresis. The electrode and the insulating material should be reasonably inert with regard to chemical reaction in aqueous alkaline solutions at elevated temperature.
Moreover, the electrode pair should not acquire a permanent residual polarization nor should the electrode pair become excessively polarized during the measurement cycle. This is assured when electrode response is very nearly independent of the frequency at which the measurement is made. There should be no significant potential drop across the electrode pair, except during the measurement cycle. In other words, the electrode, when immersed in a electrolytic solution, should not give rise to galvanic action in the solution.
Traditionally, the conductivity of boiler water has been used to provide an estimate of the dissolved ionic solids present in the boiler, which is, in turn, empirically correlated with the purity of steam exiting the boiler. Generally, the conductivity of boiler water is measured at temperatures much cooler than those experienced in the boiler. This practice is a result of the relative ease with which accurate measurements of the conductance can be made at lower (near ambient) temperatures. The ASME has established guidelines for this parameter, along with caveats concerning its use.
However, the effect of temperature on electrolytic conductivity is pronounced, especially in boiler waters where hydrolytic chemical species, both organic and inorganic, are present. The hydrolysis and dissociation reactions produce ionic species which contribute to the overall conductivity of the boiler water. These reactions exhibit equilibria which are also greatly affected by temperature, as is the dissociation equilibrium of water itself. Thus, measurement of electrolytic conductivity at the operating temperature of the boiler would provide a more accurate picture of the actual ionic condition of the boiler water.
In high purity (20-50 uS/cm), but alkaline, boiler water above ca. 600 psig (250.degree. C.), it has been generally considered infeasible to accurately measure conductivity in situ. One of the principal difficulties encountered when attempting to make measurements under these conditions (i.e., alkaline aqueous solutions at elevated temperatures) is the susceptibility of ceramic materials used for electrical insulation of the electrode pair towards dissolution. Another problem, albeit less serious, arises from the practical working temperature limit (about 250.degree. C.) of polytetrafluoroethylene, and other fluoropolymers, commonly used in the pressure seals.
Use of instrumentation, operating in situ in boiler systems, which is based on principles of electrolytic conductance is well-established. For example, there are liquid level controllers, high water alarms, and high dissolved solids alarms commonly found in boiler systems, which operate based on conductance principles. These devices are simply qualitative conductance detectors. On the other hand, the conductivity electrode of the present invention is capable of accurately measuring boiler water conductivity under operating conditions.